This invention relates to a suspension system for a vehicle having four or more wheels.
The invention has been developed primarily for use in motor cars of various types and will be described hereinafter with reference to that application. However, it will be appreciated that the invention is not limited to that particular field of use.
All vehicles that travel over, and support themselves from the ground, benefit from having a compliant suspension system. The suspension allows the vehicle and its contents to travel in a relatively smooth path while the suspension accommodates for the unevenness of the ground and keeps the wheels in contact with the road.
If a car is designed primarily for a comfortable xe2x80x9cridexe2x80x9d it is provided with soft, long travel suspension springs to absorb the bumps on the road. However, when such a car drives around a corner at speed the body xe2x80x9crollsxe2x80x9d outwards. This is uncomfortable for the passengers and can also reduce the precision with which the car can be placed on the road, thus impairing the xe2x80x9chandlingxe2x80x9d of the car. On the other hand, a car designed primarily for good handling is usually provided with stiff, short travel springs which make the car feel more responsive to the driver""s inputs, but also make the ride harsh over bumps and reduce the total xe2x80x9cgripxe2x80x9d of the tyres on the road. The choice between soft and stiff springing is often called the xe2x80x9cride-handlingxe2x80x9d compromise.
Some expressions with common but perhaps vague meanings are more closely defined below.
The term xe2x80x9ccarxe2x80x9d will be used to cover cars, trucks, trailers, caravans, etc., generally with four wheels but possibly with more.
The xe2x80x9csuspensionxe2x80x9d of the car refers to the mechanisms which connect the wheels to the xe2x80x9cbodyxe2x80x9d of the car (also called the xe2x80x9csprung-massxe2x80x9d) allowing the wheels to move in a predominantly vertical direction whilst also supporting the weight of the car. Most cars have a single xe2x80x9cspring-damperxe2x80x9d unit which controls this vertical movement of each wheel, while some cars have additional springs which are connected to more than one wheel.
A xe2x80x9cspringxe2x80x9d is any elastic element that will deflect by a predetermined and significant amount when a force is applied to it, and then return to its original length when the force is removed. A spring can be made of any of the common materials such as steel, rubber, fibreglass or compressed gas in a container. The ratio of applied force increment per deflection increment (measured in pounds-per-inch or newtons-per-meter) is called the xe2x80x9cspring ratexe2x80x9d. For most steel springs, the spring rate is constant and is determined by the size of the spring. However, due to leverages in the suspension mechanism, the deflection for a given load increment at the wheelprint, called the xe2x80x9cwheel ratexe2x80x9d, is usually different to the spring rate and can vary as the suspension deflects. This is termed xe2x80x9cvariable-ratexe2x80x9d springing.
An uncontrolled spring-mass system will oscillate indefinitely when set in motion. It is the function of the xe2x80x9cdampersxe2x80x9d to minimise the unwanted oscillations. The main characteristics of dampers is that they only exert a force on the suspension when the suspension is moving (that is, moving up or down), and the force is always in the direction opposite to that of the suspension movement. The force exerted is therefore a function of the velocity of the suspension and is almost always of variable rate.
Any solid object can move with six xe2x80x9cdegrees of freedomxe2x80x9d with respect to another solid object. For vehicles (of all types) it is usual to use a rectangular coordinate system fixed to the vehicle to describe the motion of the vehicle body with respect to the ground. The degrees of freedom are then usually named xe2x80x9cforward/backwardxe2x80x9d for longitudinal motion of the body; xe2x80x9csideslipxe2x80x9d for lateral motion of the body; xe2x80x9cbouncexe2x80x9d (or xe2x80x9cheavexe2x80x9d) for vertical motion of the body; xe2x80x9crollxe2x80x9d for rotation around a longitudinal axis through the body; xe2x80x9cpitchxe2x80x9d for rotation about a lateral axis; and xe2x80x9cyawxe2x80x9d for rotation about a vertical axis (see FIG. 1). This assumes that the body is relatively rigid. Note also that for a car travelling on an essentially flat surface only bounce, pitch and roll motions will cause vertical movements of the suspension.
While the vehicle body moves with respect to the ground, the wheels can also move with respect to the body. This movement refers only to the relative motion between the body and the contact area between wheel and ground and not to wheel rotation, steering and so on. For the purpose of this discussion, only the one degree of freedom of motion that each xe2x80x9cwheelprintxe2x80x9d has along a predominantly vertical linear path will be considered. So, for a car with four wheels there are four degrees of freedom for the motion of the four wheelprints with respect to the body. To put it another way, it would be necessary to specify four separate parameters (one for each degree of freedom) to completely define the positions of all four wheelprints with respect to the body. The simplest way to do this is to specify the vertical position of each wheelprint with respect to the body (see FIG. 2).
Another way of specifying the four wheelprint positions is shown in FIG. 3 and is called a xe2x80x9cmodalxe2x80x9d description of the suspension. Each mode (or degree of freedom) involves the movement of all four wheels. The first three modes of xe2x80x9cbouncexe2x80x9d, xe2x80x9cpitchxe2x80x9d and xe2x80x9crollxe2x80x9d are similar but opposite to the motions of the body with respect to ground. That is, as the body bounces down towards the ground, the wheelprints bounce up towards the body, and similarly for pitch and roll. These three modes are chosen because they can be directly related to the similar motions of the body.
Any combination of suspension bounce, pitch and roll will always keep all four wheelprints in a xe2x80x9cflatxe2x80x9d plane. However any stretch of road will, in practice, have some degree of unevenness. Small scale unevenness will be referred to as xe2x80x9cbumpsxe2x80x9d while larger scale unevenness of the road surface will be called xe2x80x9ctwistxe2x80x9d. Bumps are typically shorter in length than the car""s wheelbase while twist of the road surface is a bump that is longer than the wheelbase. This unevenness will inevitably try to force one wheelprint out of the plane defined by the other three wheelprints. The fourth mode of suspension movement, shown in FIG. 3d, is xe2x80x9ctwistxe2x80x9d and is the only mode that allows all four wheelprints to stay in contact with uneven ground.
The units used to measure the modes in FIGS. 3a to 3d are linear (for example, metres or inches). Angular units could be used for pitch, roll and twist. However, it is more convenient to use linear units for all the modes as it is then a simple matter of summation to calculate the heights of the individual wheelprints. The two sets of wheelprint positions shown in FIG. 2 and FIG. 3 describe the same physical situation. It is seen that a single wheel bounce is made up of one quarter contributions of the four modes of bounce, pitch, roll and twist. Also for equal loading on all wheels the single wheel spring rate is given by the summation of one-sixteenth of each of the spring rates in bounce, pitch, roll and twist.
The four modes of suspension movement of bounce, pitch, roll and twist are introduced here because they are the key to understanding the operation of the suspension system of the present invention which will be described later.
There are many aspects of a car""s design that affect its dynamic behaviour. The following section considers the contribution of the suspension, as defined above, to handling, ride and grip.
Handling is a subjective concept. It can be described as the precision of feeling that the driver has for the reactions of the car in response to the driver""s control inputs. The driver""s main inputs to the car are via the accelerator, brake and steering wheel. Therefore the main reactions of the car that the driver wants to feel are a forwards acceleration, or force, in response to the accelerator, a backwards force from the brakes, and a yawing moment followed by a lateral force from the steering. The relative magnitudes of these forces, which come mainly from the four wheelprints, are referred to as the xe2x80x9cbalancexe2x80x9d of the car. If during cornering the lateral forces acting on the front wheels are less than expected then the car is said to xe2x80x9cundersteerxe2x80x9d and conversely if the rear wheels lose grip, then the car xe2x80x9coversteersxe2x80x9d.
The reactions of the car that a driver wants to feel are mainly those that lie in the horizontal plane. Any bounce, pitch or roll motions that occur in response to the driver""s inputs are extraneous and detract from the driver""s feel of the car""s grip on the road. It can be argued that a car with ideal handling would lean into a comer like a horse, bicycle or aeroplane. This inward roll would be more comfortable, and natural, for the driver because the inertial forces on his body would be directly downwardly into the seat rather than sideways off the seat. Certainly an outward roll is undesirable because it is in the unnatural direction, and results in the driver being lifted out of the seat. Due to the width and relatively low ground clearance of modem cars, the total amount of inward roll possible during fast comers is not enough to completely balance the driver in the seat. Similar arguments can be applied to pitch during acceleration and braking. So as a first approximation, a good handling car should accelerate, brake and corner with a xe2x80x9cflatxe2x80x9d (horizontal) attitude and with minimal bounce, pitch or roll motions.
For a car to have a comfortable ride over bumps, it should have long travel softly sprung lightweight wheels that bounce vertically with no tilt of the wheel""s axle (that is, no xe2x80x9ccamber changexe2x80x9d). Any stiffness in the suspension will result in bumps causing a vertical harshness in the ride. Any excess xe2x80x9cunsprung massxe2x80x9d increases the forces necessary to accelerate the wheel up and over the bump and then down on the other side, resulting in a vertical harshness of ride. Any lateral movement of the wheels during bounce (called xe2x80x9cscrubxe2x80x9d) will cause lateral forces to act on the wheel, and any camber change will cause gyroscopic forces to act on the wheel. These lateral and gyroscopic forces will be transmitted from the wheel to the body and will be felt mainly as a horizontal harshness of ride.
A side effect of a car having soft springs and long vertical wheel travel is that it will generally have a lower rolling resistance than a stiff suspension with lots of wheel scrub. Lower rolling resistance leads to lower fuel consumption.
For a car to have good grip on the road it should maximise the amount of available rubber in contact with the road and it should equalise the pressure over all of that rubber. It is commonly thought that a sporty, stiffly sprung car with good handling will automatically have good grip. This is not necessarily so. Beyond an optimum limit, the coefficient of friction between the wheelprint and the road decreases as the pressure between the wheelprint and the road increases. Since all roads have some bumps or twist, a very stiffly sprung car (with all wheelprints fixed in a flat plane) will spend much of the time with only three wheelprints in firm contact with the ground. This means that three quarters of the total wheelprint will be carrying most of the vertical load, thereby increasing wheelprint pressures and reducing their coefficient of friction. This in turn reduces the total grip available for acceleration, braking or cornering. So for good grip on an uneven road a car needs soft springs, or more precisely a soft twist-mode spring rate, to maintain relatively constant loads on each wheelprint. Also the wheels should remain perpendicular to the road surface to maintain an even loading across the width of each wheelprint.
When a car accelerates horizontally (during either acceleration, braking or cornering) there is a rotational force created on the car by the horizontal forces acting from the road on the wheelprints (at ground level) and the inertial forces of the body which can be considered to act at the centre of gravity (always above ground level). This rotational force is called the xe2x80x9cgross pitch momentxe2x80x9d when caused by longitudinal acceleration and the xe2x80x9cgross roll momentxe2x80x9d when caused by lateral acceleration.
The above moments cause a transfer of vertical wheel loads from one end or side of the car to the other. So during braking the rear wheels will feel a reduction in vertical load while the front wheels will be subject to an increase in vertical load. Likewise, during cornering, there is a transfer of vertical load from the wheels on the inside of the comer to the wheels on the outside. The above gross moments exist regardless of the type of suspension. They are dependant only on, and are directly proportional to, the acceleration and the height of the centre of gravity.
The response of the car""s sprung mass to the above moments is called its xe2x80x9cpitch responsexe2x80x9d and xe2x80x9croll responsexe2x80x9d. Depending on the type of suspension, the sprung mass can remain substantially static during the application of these moments, or it can rotate in the direction of the moment, or it can rotate in the opposite direction to the moment, and it can also simultaneously move in the horizontal and vertical directions. FIGS. 4 and 5 show a method of estimating these responses. The xe2x80x9cpitch centrexe2x80x9d and xe2x80x9croll centrexe2x80x9d are found at the intersections of the perpendiculars to the directions of wheelprint travel, as shown.
The horizontal inertial force acting at the centre of gravity, multiplied by the vertical distance between the centre of gravity and the pitch or roll centre is called the xe2x80x9csprung mass pitch moment"" or xe2x80x9csprung mass roll momentxe2x80x9d. The sprung mass moments are thus a function of the pitch and roll centre heights and can vary enormously between different types of suspension and different displacements of a particular suspension. Note that the sprung mass moments are definitely not the same as the gross moments.
The amount of xe2x80x9cbody rollxe2x80x9d (that is, sprung mass roll) depends on the relative positions of the roll centre and the centre of gravity, and also on the roll stiffness. Roll stiffness is a measure of the rotational force on the body required to cause unit rotational displacement. It is proportional to the wheel rate and proportional to the square of the xe2x80x9ctrackxe2x80x9d (the lateral distance between wheelprint centres). If the roll centre is above the centre of gravity the top of the body will lean into comers while the centre of gravity swings outwardly. If the roll centre is below the centre of gravity, the body rolls and moves outwardly from the comer. When a car has independent suspension and a high roll centre, the cornering forces on the outer wheels are translated into an upwards force on the sprung mass. This is called xe2x80x9cjackingxe2x80x9d. This upwards force raises the body which increases the gross roll moment which can ultimately result in the whole car rolling over.
The requirements of good ride imply that the roll centre be at ground level as this results in zero scrub. However a low roll centre combined with soft springs results in significant body roll during cornering which implies bad handling. Furthermore the ride requirement of zero camber change over bumps means that as the body rolls the plane of the wheel will also roll. This reduces grip since the wheel doesn""t remain perpendicular to the ground.
Many sports cars are designed with a low ride height. The reduction in ride height reduces the height of the centre of gravity which in turn reduces the gross roll moment for a given lateral acceleration and so reduces lateral load transfer. The reduction in load transfer reduces the difference between pressures on the inner and outer wheelprints and thus increases overall grip. However, to prevent the suspension from bottoming out over bumps, the springs have to be made stiffer. The stiffer springs are also better able to resist the sprung mass roll moment so the car comers flatter with less roll and there is a feeling of improved handling. This works well on relatively flat racing tracks. However, whenever there is a significant amount of twist in the road surface (and this twist often occurs in a comer) then the stiffly sprung car loses grip, often unpredictably, which can lead to a feeling of worse handling. So even though racing car designers are not primarily concerned with a comfortable ride, they nevertheless try to use springs that are as soft as possible together with a low roll centre (to minimise jacking)xe2x80x94which inevitably results in some body roll.
Body pitch under acceleration or braking is less of a problem than body roll during cornering for the following reasons. Raising the pitch centre above ground level does not cause wheel scrub because the wheel is moving in the direction in which it is free to roll (see FIG. 4). The wheel has to roll slightly faster and then slightly slower as it goes over a bump but this is less of a problem than scrub. Also the jacking effect for a given height of pitch centre is inversely proportional to the wheelbase so the effect is less than that of roll.
Pitch stiffness is proportional to wheelbase (longitudinal distance between wheelprints) squared just as roll stiffness is proportional to track squared. Since wheelbase is typically one-and-a-half to two times the track dimension then for equal wheel rates the pitch stiffness will be two to four times the roll stiffness.
The moment of inertia (the inertial resistance of a mass to rotation) of a car body is usually several times greater in pitch than in roll. This means that there is a greater dynamic resistance from the body to pitch, say from a short stab of the brakes, than there is to roll, say, from a sharp steering movement.
Raising the pitch centre reduces the sprung mass pitch moment. The greater wheelbase increases the static resistance to pitch, and the greater pitch moment of inertia increases the dynamic resistance to pitch. Acting together, these three factors can greatly reduce the amount of pitch compared with roll.
The following notes cover some of the ways in which car manufacturers have tried to find a suitable compromise between body roll and good handling, ride and grip. The listing is roughly chronological, starting at the turn of the century.
The suspension of early model cars consists of beam axles with relatively stiff leaf springs, narrowly spaced on the axle, connected by a chassis with low torsional stiffness. The advantages of this system are that with the roll centre at about axle height and stiff springs there is little body roll. The stiff springs also give high bounce and pitch stiffness allowing for heavy loads without bottoming. However the main advantage of this system is that the twist mode is very soft so all four wheels can stay in contact with the ground over uneven, twisting terrain thus evenly distributing the load over the four wheelprints and maximising overall grip.
The disadvantages are a rough ride due to the stiff springs, and an even worse ride due to the chassis acting as an undamped spring-mass system that is excited into oscillation by every bump. The body also tends to fall apart because it is built on a flexible platform, unless the body is even more flexible than the chassis, for example in the case of a canvas canopy.
The advantages of the above system, particularly its load carrying capability, are such that it is still used by almost all trucks built today. The problems of poor ride and the body falling apart are solved by having a relatively rigid xe2x80x9ccabxe2x80x9d which is mounted on its own suspension above the front axle while the payload is carried separately over the rear axle.
Manufacturers first started fitting cars with independent suspensions in around the 1930s. Since the cars of this era were relatively tall with a large ground clearance (to cope with the rougher roads) the independent suspensions were often designed with a high roll centre to limit roll to an amount similar to that of the beam axle cars. The independent suspension cars generally rode better than the beam axle cars, mainly due to their lower unsprung masses. However the high roll centres together with the even higher centres of gravity meant that jacking (especially with rear independent suspension), and the possibility of complete rollover of the car that often accompanied it, was a real problem.
By the 1950s most manufacturers who were using independent suspensions were designing them with relatively low roll centres. Low roll centres minimise jacking, minimise scrub (which improves ride) but result in more roll during cornering.
By far the most common solution to the roll problem has been the fitting of anti-roll bars, also called sway bars or stabiliser bars (see FIG. 6). An anti-roll bar is a torsion spring connected between the front pair of wheels or between the rear pair of wheels. It works by allowing similar motions but resisting differential motions of the two wheels in the pair. So when both wheels move up or down together, the spring provides no resistance. However, when one wheel moves up and the other moves down, the spring is deflected and exerts a downwards force on the higher wheel and an equal but upwards force on the lower wheel, thereby tending to restore the two wheels to an equal height.
The advantages of anti-roll bars are that they are simple, inexpensive, easy to fit (either during manufacture or as aftermarket accessories), don""t affect the bounce mode or pitch mode spring rate, and can reduce body roll by any desired amount (a thicker bar gives less roll). The disadvantages are that they stiffen the single wheel bounce rate giving a harsher ride, and worse still, that they also act as an anti-twist mode bar. In fact, they increase the twist mode stiffness by the same amount as they increase roll stiffness. Thus, stiff anti-roll bars fitted front and rear will tend to keep all four wheelprints coplanar and thus reduce the ability of all wheelprints to stay in contact with uneven ground. This in turn reduces overall grip. In fact, if a car has an oversteering problem due to lack of grip at the rear wheels, then a common cure is to fit a stiff front anti-roll bar because this will reduce the cornering grip of the front wheels and bring the car""s handling back into balance, albeit by further compromising overall grip.
A less common solution to body roll is to fit springs that connect front and rear wheels on the same side of the car in pairs (see FIG. 7). Variations on this idea are used in the xe2x80x9cCitroen 2CVxe2x80x9d and xe2x80x9cBMCxe2x80x9d cars fitted with xe2x80x9cHydrolasticxe2x80x9d or xe2x80x9cHydragasxe2x80x9d suspensions. The interconnecting springs are arranged to allow differential motions but resist similar motions between the two wheels on each side of the car. The end result is to stiffen the bounce and roll modes while leaving the pitch and twist modes relatively soft.
The advantages of this system are that the relatively stiff roll mode reduces body roll during cornering, the soft pitch mode reduces harsh pitching motions over bumps, and the soft twist mode keeps all wheelprints in contact with uneven ground thus giving high grip. The disadvantages are that bounce stiffness increases with roll stiffness, so that high roll stiffness will give a harsh ride in bounce. Also, the distance of the interconnection is longer than for roll bars, requiring either a well laid out integrated design (2CV) or the use of hydraulics (BMC), which also means that it is not easy to fit the system aftermarket. Moreover, this system cannot readily be used to balance the handling in the same way as roll bars can.
Of course, the mode stiffnesses discussed above are relativexe2x80x94a Citroen 2CV still rolls a lot during cornering, because it was designed primarily for a soft ride. However, its pitch and twist mode stiffnesses are even softer than the bounce and roll stiffness.
Another approach, popular with some French manufacturers during the 1950""s, 60""s and 70""s, was to accept relatively large angles of body roll but to control the transient roll response with the dampers. In these systems the dampers are designed to offer a high rate of resistance to slow movements of the suspension and a lower rate to faster movements. They do this by using spring loaded valves that stay closed during slow movements but open up and reduce resistance during faster movements.
The advantages are that as the car enters a corner the relatively slow rolling motion is resisted and controlled by the dampers until the car settles gracefully to its steady state roll angle, while the faster bump movements are passed through to the springs without excess harshness.
One disadvantage of such systems is that there is excess body roll which detracts from the handling and can be uncomfortable for the passengers. Another disadvantage is that the roll can also cause the wheels to move away from being perpendicular to the road, which reduces grip.
The most sophisticated approach to resolving the ride-handling compromise are so called active suspension systems. These come in varying degrees of sophistication but typically they have sensors that detect changes in suspension position, actuators that can vary the forces on the suspension, and some kind of controller that monitors the sensors and then directs the actuators.
The simplest of these systems merely controls the damper valving in response to a driver selectable dashboard switch. The more advanced systems have a substantial power source that can rapidly move the individual wheels up and down in response to the controller""s direction. Typically the power source is an engine driven hydraulic pump with a computer controlling solenoid valves that actuate hydraulic rams which in turn move the suspension arms.
The advantage of these systems is that in principle they can deliver any sort of ride and handling behaviour that the designers desire. The major disadvantages are complexity and expense. Even though the controller costs are falling rapidly, there remains the cost of the engine driven power source, both in dollars and lost power, and reliability.
It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.
Accordingly, the invention provides a suspension system for a vehicle of the type having a body and at least four wheels independently supported for movement between upper and lower positions relative to the body, said suspension system including:
a balance mechanism including first and second balance members adapted to support suspension forces in the vehicle;
a plurality of actuating elements responsive respectively to the positions of the wheels relative to the body, a first pair of said actuating elements being operable on spaced apart points on the first balance member and a second pair of said actuating elements being operable on spaced apart points on the second balance member;
first control means for controlling relative movement between the first and second balance members of the balance mechanism;
and second control means for controlling relative movement between the balance mechanism and the body;
the first and second control means acting in combination to redistribute suspension forces between the body and the wheels, thereby to induce predetermined suspension behaviour.
Advantageously, the effective separation between the balance mechanism and the body means that there is no inherent requirement for interdependence or direct interaction between any of the four primary suspension modes of pitch, roll, twist and bounce. Consequently, these modes may be isolated and controlled substantially independently or linked in any desired combination.
Preferably, the first and second balance members are arranged such that, in the absence of supplementary suspension elements such as springs and dampers, the suspension forces applied to each of the balance members are distributed by the first control means in predetermined proportions between the associated wheels, substantially independently of the positions of the wheels relative to the body. In other words, the first control means can be configured to redistribute suspension forces between the body and the wheels, directly in response to the suspension forces acting on the wheels, largely irrespective of the extent of suspension travel within predetermined limits.
In preferred embodiments, the balance mechanism, at least to a reasonable approximation, xe2x80x9cfloatsxe2x80x9d relative to the body. Preferably also, the balance mechanism is arranged such that each mode of suspension movement tends to induce movement of the balance mechanism along or about a unique axis, thereby facilitating resolution of the suspension forces and independent control over the respective modes.
Preferably, the second control means is provided in the form of an elastic member interposed between the balance mechanism and a support frame or housing fixed to the body of the vehicle. More preferably, the second control means is elastically deformed only when the balance mechanism moves along or about one of the unique axes that is associated with either the bounce, pitch, roll or twist modal movements of the suspension. In this way, the second control means only reacts to the suspension movements of the mode uniquely associated with it and is unaffected by any of the other modal movements.
In the preferred embodiment, the balance members take the form of mechanical linkages. It will be appreciated, however, that in other embodiments hydraulic, pneumatic, electromagnetic or other forms of linkage may alternatively be used.
Preferably, the first control means include a mechanical linkage connecting the balance members. More preferably, the mechanical linkage is a fixed linkage and operates by resisting relative movement between the balance members.
More preferably, the first and second balance members, together with the first control means, are integrally formed as a single unitary balance mechanism. More preferably, the balance mechanism in this embodiment is formed as an xe2x80x9cXxe2x80x9d-shaped plate, with the first and second balance members effectively forming the respective diagonal xe2x80x9carmsxe2x80x9d of the X, and the first control means forming the substantially rigid interconnection therebetween. Of course, the plate could in practice be any physical shape. However, it can be considered at a conceptual level as an xe2x80x9cXxe2x80x9d by virtue of the manner in which the suspension forces are transferred within it.
In this way, the first control means simply functions to provide inherent resistance to relative movement between the balance members. In this embodiment, although independent relative movement between the balance members is not possible to a significant degree, some relative movement may be accommodated, if desired, by elastic deformation of the balancing plate itself. In this case, the extent of resilient deformation within the balance plate is an important design parameter in the context of the desired handling characteristics of the vehicle.
Preferably, the system includes four primary actuating elements connected proximally to the respective wheels, and four secondary actuating elements remote from the wheels and operable directly on the balance mechanism, the primary actuating elements being linked to respective secondary actuating elements by connecting means.
In one embodiment, the primary and secondary actuators include hydraulic cylinders. In an alternative embodiment, the primary and secondary actuators include mechanical linkage mechanisms, or lever arms. In a further variation, the primary and secondary actuators include a combination of hydraulic cylinders and mechanical linkages. Other forms of actuators are also envisaged including position transducers, potentiometers, pneumatic cylinders, electric motors, pressure switches, and the like. The connecting means may include hydraulic or pneumatic fluid lines or couplings, mechanical linkages, electromagnetic connections or other suitable means.
In some preferred embodiments, the connecting means permit the primary actuators at one end of the vehicle to be connected to the secondary actuators on the same end but on the opposite side of the balance mechanism, while the primary actuator associated with each of the wheels at the other end of the vehicle is connected respectively to the secondary actuator operable on the same end and on the same side of the balance mechanism.
In some embodiments, the primary and secondary actuating elements may be fixedly interconnected, or may be integrally formed.
In one preferred embodiment of the invention, suitable for use with a typical front wheel drive car, the primary and secondary actuators for each of the front wheels are hydraulic rams and are interconnected by hydraulic fluid lines. The ram for each front wheel is preferably connected with the secondary actuator on the opposite side. The primary actuators for the rear wheels, in the form of trailing arms, are preferably connected directly to their respective corners of the balance mechanism by means of respective secondary actuators in the form of ball joints. A support frame, affixed to the body, preferably carries a coil spring which acts on the balance mechanism to distribute the weight of the vehicle amongst the four wheels in a predetermined ratio. Preferably, the same support frame also carries a torsion bar, the ends of which are attached to the front and rear of the balance mechanism for moderating pitch during acceleration and braking.
Another embodiment may advantageously be used with a luxury sedan or the like wherein the front primary actuators are respectively connected to opposed corners of the balance mechanism by hydraulic means. The rear primary actuators are preferably connected to their respective secondary actuators, also by hydraulic means. The first control means acting between the balance mechanism and the body is preferably provided in the form of a longitudinally spaced pair of air springs whereby the pressure of air in the springs is dependent upon vertical displacement of the front and back of the car. This arrangement of air springs acts to support the vehicle at a constant ride height and also acts to moderate pitch during acceleration and braking.
Another embodiment is preferred for use in a racing car. In this case, ride height control means are provided in the form of a ball joint between the balance mechanism and a support frame fixed to the chassis. In this case, anti-pitch control means are preferably provided in the form of a pair of longitudinally spaced apart springs acting on the balance mechanism. These springs are preferably pre-loaded against stops to provide a predetermined response to harsh bumps once the threshold preload force is overcome whilst minimising pitch during normal acceleration and braking.
Another preferred embodiment, suitable for use in a low cost car, uses mechanical pull rods, acting between the wheel control arms and the balance mechanism, as the primary and secondary actuators. Control means for the balance mechanism are preferably provided by two telescopic dampers and a single coil spring.
In another prefered form, suitable for use in a four wheel drive vehicle, mechanical pushrods transmit the wheel loads from the wheel control arms to the balance mechanism. Control means for the balance mechanism are preferably provided by two opposed spring damper units.
Throughout the description and claims, the words xe2x80x9ccontrolxe2x80x9d, xe2x80x9ccontrolledxe2x80x9d, xe2x80x9ccontrollingxe2x80x9d, xe2x80x9ccontrol meansxe2x80x9d and the like are intended to be interpreted in a broad sense and to include, by way of example, apparatus or systems incorporating control means which are active or passive, direct or indirect, positive or resistive, movable or fixed, proportional or disproportional, digital or analogue, integral or discrete, mechanical, hydraulic, pneumatic, electromechanical, magnetic or optical.